1. Field of the Invention
The present invention relates to a method of hermetically sealing a fiber optic light guide in a metallic feedthrough sleeve by means of glass solder and to a hermetically sealed feedthrough device made by this method.
2. Description of the Related Art
Modern networks for telecommunication and internet (data highways) use fiber optic light guides, i.e. fiberglass cables, for data transmission, which comprise bundles of individual optical fibers. The fiber optic light guide has a glass core, which is surrounded by a light-tight jacket or coating.
Because of the growth of this sort of fiber optic network a great need has arisen for feedthrough devices for individual fiber optic light guides, i.e. individual optical fiber guides, for example in hybrid housings with converters for converting optical signals into electrical signals. The housing hermetically seals electro-optic components, which must be protected from the action of corrosive media, moisture and the like. These feedthrough devices must be permanently hermetically sealed for this purpose, so that these components are reliably and continuously protected.
A series of methods are already known for making hermetic feedthroughs.
In a known device for hermetically sealing a fiber optic light guide (DE 34 31 748), two feedthrough elements are provided, which must be attached and sealed to the wall and/or the fiber optic light guide and after that with each other. That is troublesome and time-consuming. It can cause a series of errors.
In another known feedthrough for a fiber optic light guide (U.S. Pat. No. 4,357,072) the exposed fiber optic light guide is metallized and is sealed with the wall with metallic solder. The metallization of a fiber optic light guide made from glass is expensive and can easily involve production of faults or errors. A protection against bending for the portion of the fiber optic light guide extending out of the feedthrough is not possible in a simple manner.
In the EP-A 0 105 198 a pressure-tight and gas-tight-fiber optic-light-guide feedthrough is described, in which a fiber optic light guide without surrounding protective layer is sealed in a feedthrough sleeve by melting or fusion of low-melting glass. It opens at respective ends into a plug connector. Two plug connectors are also provided on the entrance and outlet sides of the feedthrough sleeve for the fiber optic light guide, with which disadvantageous coupling losses unavoidably occur in or during connection of the fiber optic light guide.
In practice methods for making hermetically sealed feedthrough devices are known, in which a metallic sleeve is provided, through which a glass fiber is guided and, for example, bonded to a feedthrough sleeve by means of epoxy adhesive or by spraying liquid metal soft solder into the sleeve. In the latter case the glass fiber is first metallized. The feedthrough sleeve is then soldered to the wall of a hybrid housing. This process, above all the adhesive techniques, however produces no permanent hermetic seal.
It is also known (EP 0 332 046 B1), to attach and seal the through-going fiber optic light guide in a feedthrough sleeve by means of glass solder and to hermetically seal the feedthrough sleeve itself similarly in a wall opening by means of glass solder.
Also EP 0 274 222 discloses a feedthrough with a metallic sleeve, in which the fiber optic light guide is attached by means of glass solder.
This known glass solder process includes sealing by means of glass solder capillaries. No satisfactory results are produced with this process, because of significant mismatching of the thermal expansion coefficients. These glass solder capillaries are only made from stable glass solder. They have comparatively high thermal expansion coefficients, usually greater than 10 ppm/K. Melting temperatures, which lead to extreme thermal damage (brittlization) of the fibers, were required for capillaries made of drawable stable glasses with small thermal expansion coefficients, e.g. xcex1≅5 ppm/K. Because of that first no worthwhile hermetic seal is possible in a metallic sleeve with xcex1≅5 ppm/K, i.e. in a sleeve made of a material, which is preferred for electro-optic housings because of its small thermal expansion properties. Also the high diathermic mismatchxe2x80x94dependent on stressesxe2x80x94causes attenuation of optical signals.
It is an object of the present invention to provide a method of hermetically sealing a fiber optic light guide in a metallic feedthrough sleeve with glass solder, which produces a permanent hermetically sealed feedthrough device and is performed with comparatively simple process technology.
It is a further object of the present invention to provide a suitable permanent hermetically sealed feedthrough device in a comparatively simple manner.
According to the invention this object is attained by a method for hermetically sealing a fiber optic light guide in a metallic feedthrough sleeve by means of glass solder, which comprises the steps of:
a) preparing a feedthrough sleeve consisting of a metallic material, which comprises an invar alloy;
b) removing at least one protective layer from the fiber optic light guide to expose a glass fiber;
c) preparing two half-cylinder-shaped pressed parts made from a low-melting composite-glass solder with thermal expansion properties adapted to the material of the feedthrough sleeve, the half-cylinder-shaped pressed parts having respective axial grooves with corresponding cross-sections corresponding to a diameter of the exposed glass fiber;
d) assembling both half-cylinder-shaped pressed parts together with the exposed glass fiber in the axial grooves of the half-cylinder-shaped pressed parts; and
e) melting the half-cylinder-shaped pressed parts with each other together with the glass fiber and the feedthrough sleeve by local heating.
In regard to the hermetic sealing of the fiber optic light guide in the metallic feedthrough sleeve by means of glass solder, the method according to the invention is successful. In this method the feedthrough sleeve is made of an invar alloy and the glass solder from the low-melting composite-glass solder with a thermal expansion that corresponds to that of the material of the feedthrough sleeve. The feedthrough sleeve is formed from the prefabricated half-cylinder-shaped pressed parts of low-melting glass solder, between which the optical fiber is embedded after removal of its protective jacket or layers.
Because of the features of the inventive method it is comparatively easy to provide a permanent hermetically sealed feedthrough device.
Additional features of preferred embodiments are set forth in the dependent claims appended hereinbelow.
In a particularly preferred embodiment the feedthrough sleeve is provided with a first axial section having a first interior diameter, which is filled with adhesive around the fiber optic light guide and the at least one protective layer is removed from only a portion of the fiber optic light guide in the first axial section. The feed through sleeve has a second axial section having a second interior diameter for receiving the half-cylinder-shaped pressed parts, this second interior diameter being greater than the first interior diameter.
In further preferred embodiments the Invar alloy consists of a Nixe2x80x94Coxe2x80x94Fe alloy, a Fexe2x80x94Nixe2x80x94Cr alloy or a Fexe2x80x94Ni alloy. Gold coating the feedthrough sleeve facilitates sealing the optical fiber light guide in it with the glass solder.
Advantageously the half-cylinder-shaped pressed parts are made with a process according to press/sinter technology. In this process the half-cylinder-shaped pressed parts of the composite-glass solder can be made from a lead-borate glass with an inert expansion-lowering filling material, especially xcex2-Eucryptite. Alternatively, they can be made from a composite-glass solder consisting of a phosphate glass with an inert expansion-lowering filling material.
The local heating for local thermal sealing takes place by means of an electrically heated incandescent filament, inductively, by means of focussed infrared radiation or in an oven.
The hermetically sealed feedthrough device for hermetically sealing a fiber optic light guide according to the invention comprises a metallic feedthrough sleeve consisting of a metallic material, which comprises an invar alloy, and prefabricated half-cylinder-shaped pressed parts consisting of low-melting composite-glass solder, which have thermal expansion properties adapted to the metallic material of the feedthrough sleeve. A portion of the fiber optic light guide is sealed in this feedthrough device between these prefabricated half-cylinder-shaped pressed parts after removal of at least one protective layer or coating provided on this portion of the fiber optic light guide.
The low-melting composite-glass solder preferably has a thermal expansion coefficient of from 4.3 to 5 ppm/K and a processing temperature during sealing of under a Curie temperature of the invar alloy.